Transmission lines carry electric energy from one
point to another in an electric power system. They can carry alternating
current or direct current or a system can be a combination of both. Also,
electric current can be carried by either overhead or underground lines. The
main characteristics that distinguish transmission lines from distribution
lines are that they are operated at relatively high voltages, they transmit
large quantities of power and they transmit the power over large distances.

Overhead AC transmission lines share one characteristic;
they carry 3-phase current. The voltages vary according to the particular
grid system they belong to. Transmission voltages vary from 69 kv up to 765
kv. The following are examples of different overhead transmission line
structures in use today. The DC voltage transmission
tower has lines in pairs rather than in threes (for 3-phase current)
as in AC voltage lines. One line is the positive current line and the other
is the negative current line.

Subtransmission lines carry voltages reduced from the
major transmission line system. Typically, 34.5 kv to 69 kv, this power is
sent to regional distribution substations. Sometimes the subtransmission
voltage is tapped along the way for use in industrial or large commercial
operations. Some utilities categorize these as transmission lines.

Figure 13. Transmission lines are installed in a tunnel,
which enables many circuits in a limited area.

Figure 14. Transmission line laid in a trench

Figure 15. Covering the transmission line

Figure 16. Underground transmission line tunnel

Figure 17. Heat is generated when electricity flows through cables, limiting the power transmission capacity in tunnels. To increase the capacity, a
tunnel cooling system can be installed. The system above circulates cold water through the tunnels.